This invention relates to a method of detecting failure of a valve timing changeover mechanism in a valve timing changeover control system of an internal combustion engine which changes the valve timing of inlet and/or exhaust valves.
It is conventionally known e.g. from Japanese Patent Publication (Kokoku) No. 49-33289 to provide a valve timing changeover mechanism which changes the valve timing of inlet and exhaust valves of an internal combustion engine, in response to an instruction signal from a control unit depending on operating conditions of the engine. (Throughout the specification and claims, "the changeover of valve timing" means changeover of a valve lift amount alone, or changeover of both the valve lift amount and at least one of the valve opening period and the valve opening angle. Further, "the changeover of valve timing" includes inhibiting operation of one of a plurality of inlet or exhaust valves provided for each cylinder of the engine in a specific operating region of the engine.)
In such an engine capable of changing the valve timing, if the valve timing changeover mechanism becomes faulty, there can be a case in which even if the control unit outputs an instruction signal for changeover of the valve timing, the valve timing is not actually changed in spite of the instruction signal. In such a case, other controls (fuel supply control, ignition timing control, etc.) which are carried out in accordance with the valve timing controlled in response to the instruction signal are not carried out in manners suitable for the actually selected valve timing, which results in degraded performance of the engine such as degraded driveability. In order to solve this problem, a failure detecting device for detecting failure of a valve timing changeover mechanism has already been proposed in Japanese Provisional Patent Publication (Kokai) No. 2-102310 by the present assignee.
The proposed failure detecting device is adapted to detect failure of a valve timing changeover mechanism which comprises first and second cams having profiles different from each other, and first and second rocker arms on which the first and second cams slide, respectively. The cams for driving inlet and exhaust valves are changed over between the first and the second cams to thereby change the valve timing between low speed valve timing suitable for a lower engine rotational speed range and high speed valve timing suitable for a higher engine rotational speed range. The proposed failure detecting device is based upon the fact that if the valve timing is actually changed in response to the instruction signal from the control unit, the states of contact between the first and second cams and the respective first and second rocker arms are changed. Therefore, a value of electric resistance between the rocker arms and a cam shaft on which the cams are integrally formed is detected, and based on the detected electric resistance value and the instruction signal, it is detected whether there is failure in the valve timing changeover mechanism.
However, the proposed failure detecting device requires application of a voltage between the cam shaft and the rocker arms in order to detect the electric resistance value therebetween, so that it is necessary to electrically insulate the cam shaft and rocker arms from the cylinder block of the engine, which results in a more complicated structure of the engine. Further, the cam and rocker arms have to be formed of materials having higher electric resistance, which brings about the problems of the effective lives of the component parts and the manufacturing cost thereof.